Amplification of Dissymmetry Factors in π-Extended [7]- and [9]Helicenes

Abstract

π-Extended helicenes constitute an important class of polycyclic aromatic hydrocarbons with intrinsic chirality. Herein, we report the syntheses of π-extended [7]helicene 4 and π-extended [9]helicene 6 through regioselective cyclodehydrogenation in high yields, where a "prefusion" strategy plays a key role in preventing undesirable aryl rearrangements. The unique helical structures are unambiguously confirmed by X-ray crystal structure analysis. Compared to the parent pristine [7]helicene and [9]helicene, these novel π-extended helicenes display significantly improved photophysical properties, with a quantum yield of 0.41 for 6. After optical resolution by chiral high-performance liquid chromatography, the chiroptical properties of enantiomers 4-P/M and 6-P/M are investigated, revealing that the small variation in helical length from [7] to [9] can cause an approximately 10-fold increase in the dissymmetry factors. The circularly polarized luminescence brightness of 6 reaches 12.6 M^-1 cm^-1 as one of the highest among carbohelicenes.

Scheme 1. π-Extended Helicenes and Their CPL Properties

Scheme 2. Illustration of the Prefusion Strategy To Prevent Aryl Rearrangement and Achieve the Desired π-Extended Helicenes 4 and 6

Figure 1

(A) Synthetic route toward 4 and 6. (B and C) Aromatic regions of the ¹H NMR spectra of 4 and 6 with peak assignments.

Figure 2

Single-crystal structures of (A and B) 4-M and (C and D) 6-M. (E and F) Molecular packing of 4 and 6. All hydrogen atoms and the tert-butyl groups in (E and F) are omitted for clarity. The P- and M-enantiomers are highlighted in blue and red, respectively.

Figure 3

(A) Absorption spectra and (B) CD spectra of 4 and 6 in THF solutions. Solution concentration: 10^–5 M. (C and D) Transition dipole moments of (C) 4-P and (D) 6-P for the S₀ → S₁ transition. The electric transition dipole moments (μ) are shown in blue, and the magnetic transition dipole moments (m) are shown in red. The length of the m vector is amplified 200 times for clarity.

Figure 4

(A and B) CPL emission spectra and (C) luminescence dissymmetry factors of 4-P/M and 6-P/M in THF. Concentration: 10^–5 M. Excitation: 380 nm for 4 and 425 nm for 6.